Although the proliferation of digital signal processing equipment has met with widespread acceptance in a variety of industries, telephone companies have been slow to convert to or integrate digital signalling subsystems and communication schemes into their well established copper wire networks. One of the principal reasons for such reticence is the fact that a significant part, if not all, of an established telephone network employs analog signalling equipment. Consequently, to be accepted by the telephone company, any digital product must not only be a cost effective replacement for existing circuitry, but it must be signal-compatible with any remaining analog units of the network to which it may be interfaced.
Most telephone networks currently utilize digital signalling highways to carry voice and data traffic between offices, via a pulse code modulated (PCM) carrier (such as a T1 carrier having a data rate of 1.544 Mb/s), onto which a plurality of (e.g. 24) data channels are multiplexed into serially transmitted frames of digital data. Since the successful completion of a telephone call requires the operation of telephone equipment of the calling and called parties, it is also necessary to include, as part of the communication traffic between offices, additional signals through which such equipment is controlled.
In a conventional analog environment, such control signalling is realized by the application of prescribed voltages to respective (e.g. tip and ring) leads of a (two or four-wire) analog channel. Because such analog signalling is not possible over a digital link, T1 protocol customarily incorporates the necessary signalling information in the form of control bits, termed A and B bits, into the frames of voice/data traffic carried by the digital communications channel. One accepted industry practice of inserting such control bits into the digitized voice traffic without noticeable corruption of the voice signals is to `rob` the least significant bit of one or more bytes of a data frame and replace the `robbed` data bit with a control bit.
In the course of interfacing these frames of digital data at an inter-station location within the T1 link, such as at an intermediate central office, it has been conventional practice to port respective ends of the digital (T1) link to dedicated tandem digital/analog and analog/digital interface pairs, which are interconnected by way of an analog, cross-connect link, thereby permitting both access to and testing of the central office's equipment by a standard switched maintenance access system (SMAS) or a switched access remote test system (SARTS). Unfortunately, because the digital communication equipments at the respective ports of the intra-office, cross-connect link are mutually asynchronous, it has been necessary to provide an intermediate analog highway in the central office for effecting the transmission of the control information carried by the A and B bits onto the next T1 link.
More particularly, in a leased line application, which conveys only voice or data traffic, a frame of digital data contains no signalling or control information (A and B bits) that must be extracted from the data stream, so that it is possible to successfully cross-connect the respective T1 ports of the intermediate central office by a digital-only link. However, since a typical T1 channel also can be expected to carry signalling traffic in addition to purely voice/data bytes, some means must be provided to ensure successful transmission and synchronization of the information contained in the signalling bits over the intra-office cross-connect link. As noted above, one conventional technique to handle this problem is to use a separate, dedicated analog pair which carry battery or ground potentials for auxiliary signalling in addition to four-wire link for voice/signalling traffic.
A shortcoming of a conventional `analog` cross-connect (a traditional motive for which is its ability to accommodate access from analog test equipment, as noted previously) is the fact that all of the voice traffic is subject to potential data corruption by quantization errors that may be introduced in the course of the analog-to-digital conversion of the out going T1 data.